Cobalt and Black Stains

Looking for a stain recipe, that doesn't settle out, to paint consistently opaque outlines of shapes on pieces already glazed with a semi-transparent glaze, then fired in oxidation at cone 5--for both black and cobalt blue stains. Can someone point me in the right direction? Thanks.

I use a mixture of cobalt oxide, bentonite, vinegar and a little bit of dilute hydrochloric acid (like a couple drops)
I never use a recipe for this stain because it is pretty much all cobalt. i just add bentonite until it quits wanting to settle quickly.
This same idea works with other oxides i am sure, i just havent used any.

Newmarket;
Here are a couple of recipes designed for on-glaze painting, on the unfired glaze surface.[pot is already bisqued, and raw glaze applied.]
These are for Cone 10, but should probably work for you.
As with everything on this blog, test ,test,test.
Tom's Blue Pig.[mine]
Cobalt Carb. 20
Tin Oxide 20
Mang. Diox. 10
Red Art 15
China clay[E.P.K.]20
Red Iron Ox. 5
Total 100

Newmarket;Here are a couple of recipes designed for on-glaze painting, on the unfired glaze surface.[pot is already bisqued, and raw glaze applied.]These are for Cone 10, but should probably work for you.As with everything on this blog, test ,test,test.Tom's Blue Pig.[mine]Cobalt Carb. 20Tin Oxide 20Mang. Diox. 10Red Art 15China clay[E.P.K.]20Red Iron Ox. 5Total 100

I use a mixture of cobalt oxide, bentonite, vinegar and a little bit of dilute hydrochloric acid (like a couple drops)I never use a recipe for this stain because it is pretty much all cobalt. i just add bentonite until it quits wanting to settle quickly.This same idea works with other oxides i am sure, i just havent used any.

I add colorants to terra sig. Settling is a constant problem and really complicates using the TS. Would adding bentonite help?

Newmarket;Here are a couple of recipes designed for on-glaze painting, on the unfired glaze surface.[pot is already bisqued, and raw glaze applied.]These are for Cone 10, but should probably work for you.As with everything on this blog, test ,test,test.Tom's Blue Pig.[mine]Cobalt Carb. 20Tin Oxide 20Mang. Diox. 10Red Art 15China clay[E.P.K.]20Red Iron Ox. 5Total 100

Newmarket;Here are a couple of recipes designed for on-glaze painting, on the unfired glaze surface.[pot is already bisqued, and raw glaze applied.]These are for Cone 10, but should probably work for you.As with everything on this blog, test ,test,test.Tom's Blue Pig.[mine]Cobalt Carb. 20Tin Oxide 20Mang. Diox. 10Red Art 15China clay[E.P.K.]20Red Iron Ox. 5Total 100

If you look at the plate stuck to my face, you will see that I actually use on-glaze decoration.Tom Roberts

Are these food-safe? Love that plate!

That "is this food safe" question comes up all the time in so many contexts. One of the HUGE difficulties in answering this kind of thng accurately is that there is no formal legal definition that precisely defines that term, "food safe". There are US FDA standards for the leaching of lead and cadmium compounds out of fired glazes. The State of California has similar but more stringent rules for those same compounds. The commercial food service industry has various standards for commercially used food production and service wares. State and local areas have regulations for the restaraunt use of containers. And so on.

That being said, most folks think of "food safe-ness" as not leaching anything potentially toxic into food substances stored or served on or in the ceramic container.

If that is the yardstick used, then you have to first get into the potential effects of the various compounds that might be leached into foods from fired glazes. That alone is a hugely complicated subject. Different compounds present different levels of hazard at different concentrations. Because of things like the concept of "total body burden", levels of potential toxins that might not harm 99.9999999% of the population might cause issue with others.

There are the "special cases" out in the world. For most people, copper compounds are pretty much safe, although they can affect the taste of some foods. But for some with a particualr rare disease... a little copper ingested can kill them. So there are no absolutes here.

Many people use the US drinking water standards in combination with a laboratory grade standard acetic acid leaching test (used for lead and cadmium) to use as a "guiderline" to help them determine this concept. But that is not a "law" ... it is simply a handy guideline. And remember that the US drinking water standards are based on consuming a HUGE amount of water containing the aforementioned chemical in a day/week/year.... so usuing this as a guide has a HUGE margin of safety.

But remember.... the ONLY ceramic related substances that are formally regulated in the USA are lead and cadmium. Even if the drinking water standard allows only a small amount of a chemical to be present.... there is no formal regulation or law that ties this directly to ceramic objects you might produce.

However,.... that being said... there is the "Law of Merchantability". That basically says that an object sold must be suitable for use for the intent for which it is sold. Defining that would be the provence of (expensive) teams of lawyers. If you make something that harms someone, and that fact can be demonstrated in a court of law, then you have liability for the damages and potentialy punitive damages (if you acted in clear disregard for information that you knew or should have known).

Now with that out of the way, we can come to the more techincal stuff.......

Those colorant washes, used either over or under the raw glaze layer, are supplying various oxides. How those oxides will get incorporated into the glass matrix of the underlying glaze first of all depends on the exact chemical composition of the glaze you might use. This also has HUGE potential variations. Some glasses "hold" certain oxides better than others. So the chemical composition of the fired glaze (the formula) needs to be considered as to its ability to resist leaching in general.

Then there is the variability of the composition of the glass formed right whree the wash is brushed or sprayed (or whatever) onto the wares. The exact thickness of the application will vary the chemical composition of the glass right in that area. Where the brushwork (or spraying or wehatever) has laid down a thinner application of the oxide wash... the glaze might be able to "hold" the colorants (and other oxides) that this application is adding into the base glass formula. But in an area where the application was thicker... it might not.

Then there is the amount of the actual surface area of the food contact surface that this colorant wash addition actually occupies as a percentage of the surface total area fo the vessel. The more surface that contains the potential material to leach out, the greater the potential for leaching.

This is WAY complicated to give a blanket accurate answer.

So what oxides in the above listed washes are of the most concern? Cobalt compounds. Manganese compounds. For some with a certain rare disease... iron compounds.

I can say with some certainty here that if the application of a wash produces a really dark, DARK color (like a blue that is almost black), and particulary if it is going to a black color (or of even more concern a metallic mattish black) then the likelihood that the glaze will leach something is greatly increased.

High saturations of colorant oxides are notoriously hard to keep in solution in a glass. The metallic surfaces you see are usually microcrystalline silicates (stained with the coloring metals) that are floating on the surface of the glass....... formed on the cooling of the frozen glaze surface as the kiln cools. They are not bonded into the glass matrix much at all.

Now with all this being said...... the level of hazard to the consumer from this kind of thing can (and often is) be severely overplayed. The leaching levels form most decent formulation liner-type glazes with a light use of such brushwork and such are going to likely test WAY low of the (VERY concervative) drinking water standards. Are there glazes and techniques that should likely not be used ofor food contact surfaces.... for sure. But using some even basic knowledge of ceramic chemistry should guide you in the right direction.

That being said... if a person does not have even a very basic understanding of ceramic chemistry and kiln firing and what is going on there, then maybe they should not be producing food service wares until they do havee this knowledge base. Or carefully stick with known decorating preparations (based upon the information from people who DO know this stuff) that produce acceptable levels of risks. They should be taking great care in just collecting glaze recipes and such and then using them.

As with the answers I often give to other ceramic technical questions .... (that is .... "test, test, test")............. that answer here is exactly the same.

But in this case, the testing is performing formal commercial lab leach testing. Yes, it will cost you about $30-50 a pop for a particualr testing regimine for a single oxide being scanned (add about $10 per extra oxide). But if you want to know, this is the ONLY way to know something accurate for sure. And the test will only give you results for a particualr glaze and wash combination, and only for a given application technique and thickness.

Hope this information is of use.

best,

......................john

PS: There is a reason that at New Hampshire Institute of Art we require all of our BFA students to take a 3 credit ceramic materials chemistry course ;)src="http://ceramicartsda...ault/wink.gif"> . I routinely cover the subjects of toxicology and glaze stability and the like.

John;
Thank-you for taking the time to answer that question about food safety. I have to admit that no one has ever asked me about my stoneware and porcelain being food safe.And I have being doing this for 35 years. I have had my Majolica tested and it passed. I do not use any lead or lead bisilicates.
I guess the easy answer would be to avoid contact with food, so maybe decorating the surface of a plate is not the best idea. The reason there is manganese in the first recipe is to mute down the nasty blueness of the cobalt. I have used that blue pigment for a long time.
I guess I need some advice here.
Tom.[TJR].

Tom do you have a hint for making the raw glaze less fluffy thus making it easier to apply the stains. I find my brush tends to get cloggy with picked up glaze.Babs

BABS;Some glazes work well for brushing on to the surface-some don't. My glazes tend to be on the matt side, and consequently contain more clay, so the surface is not powdery. You could try adding 3% Bentonite to your glaze. This will add some clay content.TJR.

Newmarket;Here are a couple of recipes designed for on-glaze painting, on the unfired glaze surface.[pot is already bisqued, and raw glaze applied.]These are for Cone 10, but should probably work for you.As with everything on this blog, test ,test,test.Tom's Blue Pig.[mine]Cobalt Carb. 20Tin Oxide 20Mang. Diox. 10Red Art 15China clay[E.P.K.]20Red Iron Ox. 5Total 100

If you look at the plate stuck to my face, you will see that I actually use on-glaze decoration.Tom Roberts

Are these food-safe? Love that plate!

That "is this food safe" question comes up all the time in so many contexts. One of the HUGE difficulties in answering this kind of thng accurately is that there is no formal legal definition that precisely defines that term, "food safe". There are US FDA standards for the leaching of lead and cadmium compounds out of fired glazes. The State of California has similar but more stringent rules for those same compounds. The commercial food service industry has various standards for commercially used food production and service wares. State and local areas have regulations for the restaraunt use of containers. And so on.

That being said, most folks think of "food safe-ness" as not leaching anything potentially toxic into food substances stored or served on or in the ceramic container.

If that is the yardstick used, then you have to first get into the potential effects of the various compounds that might be leached into foods from fired glazes. That alone is a hugely complicated subject. Different compounds present different levels of hazard at different concentrations. Because of things like the concept of "total body burden", levels of potential toxins that might not harm 99.9999999% of the population might cause issue with others.

There are the "special cases" out in the world. For most people, copper compounds are pretty much safe, although they can affect the taste of some foods. But for some with a particualr rare disease... a little copper ingested can kill them. So there are no absolutes here.

Many people use the US drinking water standards in combination with a laboratory grade standard acetic acid leaching test (used for lead and cadmium) to use as a "guiderline" to help them determine this concept. But that is not a "law" ... it is simply a handy guideline. And remember that the US drinking water standards are based on consuming a HUGE amount of water containing the aforementioned chemical in a day/week/year.... so usuing this as a guide has a HUGE margin of safety.

But remember.... the ONLY ceramic related substances that are formally regulated in the USA are lead and cadmium. Even if the drinking water standard allows only a small amount of a chemical to be present.... there is no formal regulation or law that ties this directly to ceramic objects you might produce.

However,.... that being said... there is the "Law of Merchantability". That basically says that an object sold must be suitable for use for the intent for which it is sold. Defining that would be the provence of (expensive) teams of lawyers. If you make something that harms someone, and that fact can be demonstrated in a court of law, then you have liability for the damages and potentialy punitive damages (if you acted in clear disregard for information that you knew or should have known).

Now with that out of the way, we can come to the more techincal stuff.......

Those colorant washes, used either over or under the raw glaze layer, are supplying various oxides. How those oxides will get incorporated into the glass matrix of the underlying glaze first of all depends on the exact chemical composition of the glaze you might use. This also has HUGE potential variations. Some glasses "hold" certain oxides better than others. So the chemical composition of the fired glaze (the formula) needs to be considered as to its ability to resist leaching in general.

Then there is the variability of the composition of the glass formed right whree the wash is brushed or sprayed (or whatever) onto the wares. The exact thickness of the application will vary the chemical composition of the glass right in that area. Where the brushwork (or spraying or wehatever) has laid down a thinner application of the oxide wash... the glaze might be able to "hold" the colorants (and other oxides) that this application is adding into the base glass formula. But in an area where the application was thicker... it might not.

Then there is the amount of the actual surface area of the food contact surface that this colorant wash addition actually occupies as a percentage of the surface total area fo the vessel. The more surface that contains the potential material to leach out, the greater the potential for leaching.

This is WAY complicated to give a blanket accurate answer.

So what oxides in the above listed washes are of the most concern? Cobalt compounds. Manganese compounds. For some with a certain rare disease... iron compounds.

I can say with some certainty here that if the application of a wash produces a really dark, DARK color (like a blue that is almost black), and particulary if it is going to a black color (or of even more concern a metallic mattish black) then the likelihood that the glaze will leach something is greatly increased.

High saturations of colorant oxides are notoriously hard to keep in solution in a glass. The metallic surfaces you see are usually microcrystalline silicates (stained with the coloring metals) that are floating on the surface of the glass....... formed on the cooling of the frozen glaze surface as the kiln cools. They are not bonded into the glass matrix much at all.

Now with all this being said...... the level of hazard to the consumer from this kind of thing can (and often is) be severely overplayed. The leaching levels form most decent formulation liner-type glazes with a light use of such brushwork and such are going to likely test WAY low of the (VERY concervative) drinking water standards. Are there glazes and techniques that should likely not be used ofor food contact surfaces.... for sure. But using some even basic knowledge of ceramic chemistry should guide you in the right direction.

That being said... if a person does not have even a very basic understanding of ceramic chemistry and kiln firing and what is going on there, then maybe they should not be producing food service wares until they do havee this knowledge base. Or carefully stick with known decorating preparations (based upon the information from people who DO know this stuff) that produce acceptable levels of risks. They should be taking great care in just collecting glaze recipes and such and then using them.

As with the answers I often give to other ceramic technical questions .... (that is .... "test, test, test")............. that answer here is exactly the same.

But in this case, the testing is performing formal commercial lab leach testing. Yes, it will cost you about $30-50 a pop for a particualr testing regimine for a single oxide being scanned (add about $10 per extra oxide). But if you want to know, this is the ONLY way to know something accurate for sure. And the test will only give you results for a particualr glaze and wash combination, and only for a given application technique and thickness.

Hope this information is of use.

best,

......................john

PS: There is a reason that at New Hampshire Institute of Art we require all of our BFA students to take a 3 credit ceramic materials chemistry course ;)src="http://ceramicartsda...ault/wink.gif"> . I routinely cover the subjects of toxicology and glaze stability and the like.

Thanks, John. I asked because the oxides, particularly the manganese and cobalt, are being painted on over the glaze. I would assume the end result is different than the same colorants under a clear glaze or as colorants in a glaze, but perhaps not. I love those oxides, but I have all due respect for their toxicity.

Tom do you have a hint for making the raw glaze less fluffy thus making it easier to apply the stains. I find my brush tends to get cloggy with picked up glaze.Babs

You could try adding a binder to the glaze formulation. This wild bind the glaze to the pot and make the glaze surface harder and less likely to smear. Most burn out in firing. I have used CMC gum but you should talk to your ceramic materials supplier about all the various binder additives available to find what might work best for you.Also, brushing stains can benefit from using some type of gum additive to help keep them flowing.

1% dry CMC or a thick wet solution of CMC added to your glaze should take care of the issue you are describing. it kinda forms a "candy coat" or like a "snail trail" hardness to your unfired glaze surface, so less picking up from your detail brush - also helps keep dry glaze from powdering off on your fingers when loading kiln.